It requires root. You can first create a new user name in Solaris that is the same as the new instance name you will use. For convenience, you can use the same password as the default instance id. The db2icrt command will use the directory path of the new instance name.
I would first edit the etc/services file and create the port numbers, and then reference the new port number in using the -p parmeter.
The net result is that the new instance will have a new instance owner id (different from the default instance) and a new port number. This will help you know what instance you are logged on to, and make it easy to catalog the TCP/IP node remotely to get to the correct instance.